It is known that hydroxydiphenyl can be prepared in the gas phase by catalytic dehydrogenation of compounds or mixtures of compounds consisting of completely and/or partly hydrogenated hydroxydiphenyl. Dehydrogenation catalysts which comprise nickel, chromium, aluminium, copper and an alkali metal oxide or carbonate, and if appropriate silver, such as are described, for example, in German Patent Specification 1,108,221 and German Offenlegungsschrift 2,049,809, are used here.
A process for the preparation of 2-hydroxydiphenyl by dehydrogenation of cyclohexanone derivatives in the presence of a supported catalyst is known from German Offenlegungsschrift 2,211,721. The catalyst employed in this process is prepared by depositing a very small amount of palladium, platinum, iridium or rhodium or a mixture of two or more of these elements on a support, such as silica, aluminium oxide, silica/aluminium oxide or active charcoal, and by furthermore adding a suitable amount of an alkali.
In the Offenlegungsschrift mentioned, it is emphasised especially that platinum and palladium have a particularly high activity for selective formation of the desired product. However, the noble metal catalysts employed in this process have either too low a selectivity (in particular if a rhodium-containing catalyst is used), an inadequate activity or too short a life.
It is furthermore known from German Offenlegungsschrift 2,049,809 that hydroxydiphenyl can be prepared by catalytic dehydrogenation of compounds and/or mixtures of compounds, which consist of completely and/or partly hydrogenated hydroxydiphenyl in the presence of a dehydrogenation catalyst comprising nickel, chromium, aluminium, copper and an alkali. U.S. Pat. No. 4,060,559 describes a process for the preparation of 2-hydroxydiphenyl by dehydrogenation of cyclohexylphenol, in which the reaction is carried out in the presence of a supported catalyst comprising platinum or palladium. The relatively short life of the catalysts and the sometimes inadequate activity are also disadvantages of this process.
According to EP 208,933, hydroxydiphenyl can be obtained by dehydrogenation of cyclohexylidene-cyclohexanone if a Cr-Mn-Al.sub.2 O.sub.3 catalyst comprising rhodium is used. However, the yields are in need of improvement for industrial production of hydroxydiphenyl.